Abstract

Abstract
Grunerite, Fe7Si8O22(OH)2, has been synthesized from oxide-gel and crystalline starting materials seeded with natural grunerite in a piston-cylinder apparatus at P = 12-20 kbar and T = 490-620ºC, under the fO2 of the Ni-NiO, wuestite-magnetite, and iron-wuestite buffers. The reaction curve: grunerite = 7 ferrosilite + quartz + H2O has been reversed in the 12-20 kbar range on the NNO and IW buffers. Starting materials were synthetic ferrosilite, grunerite, and quartz, and natural grunerite (Fe/(Fe + Mg) = 0.89). The reaction curve: 2 grunerite = 7 fayalite + 9 quartz + 2 H2O has been reversed at 2 kbar under QFM-conditions in cold-seal apparatus. Starting materials were synthetic fayalite and natural quartz and grunerite (Fe/(Fe + Mg) = 0.985). The high and low pressure brackets can be fitted using thermodynamic data for fayalite, ferrosilite, and quartz from Berman (1988) and the following 1 bar, 298 K data for grunerite (per gfw): V = 278.0 cm3, Sº = 725 JK-1 and ΔHf° (from elements) = -9623 kJ. Grunerite has a maximum thermal stability of 650(± 20)°C and 9.7 (± 1) kbar at an invariant point on the reaction curve: 2 ferrosilite = fayalite + quartz. This temperature is 30°C higher than that calculated by Fonarev (1987), but significantly lower than the values proposed by Miyano & Klein (1986) and Anovitz et al. (1988). Application of the grunerite data to natural Mg-bearing grunerite and cummingtonite parageneses must allow for the strong compositional stabilisation of grunerite with respect to both olivine and orthopyroxene when Mg substitutes for Fe. The lowering of the thermal stability limit of grunerite in comparison to previous studies results in considerably smaller estimates of the CO2-content of the fluid phase accompanying the high-grade metamorphism of iron formations.